The invention relates to alternate twist plied yarn where the singles strands are twisted in the same direction and are brought together and allowed to spontaneously ply together until the singles twist torque is balanced by the ply twist torque. The single strands are bonded together in the region where the singles twist reverses and they may be bonded in the plied yarn before the singles twist is reversed.
U.S. Pat. No. 4,873,821 describes a process where the alternate ply twisted yarns are bonded in the ply twisted condition before the singles twist is reversed. These yarns can be made with very short bonds (less than 5 times the plied yarn diameter) since there is at least one good crossover where strong bonding can occur. The singles twist reversal length in such yarns is very short (less than 1 times the plied yarn diameter) and it occurs at one end of the bond. Yarns made in this manner, however, are allowed to spontaneously ply together at a short distance from the exit of the twisting means so there is no significant distance over which any variations in singles twist can equalize. It has been found to be difficult to uniformly produce singles twist along the strand length since the twisting means most often employed are friction devices or fluid jet devices that inherently have relative slippage with the yarn and therefore have some variability in their twisting effect on the singles strands. The speed of the singles strands through the twisting means may also be variable so even a constant twisting rate results in uneven distribution of twist along the length of the singles yarn. Variations in singles twist results in variations in ply twist when two twisted singles are allowed to spontaneously ply together. This may produce defects in the form of sections of ply twist that vary excessively above or below the average ply twist of the yarn. When used to make carpets, yarns having sections of excessively low or high ply twist may appear as streaks in the carpet. There is also a defect called flashes caused by excessive twist imbalance in the yarn where at least one strand has a high singles twist and the other strands have low singles twist or vice-versa. This defect can be visually detected in the plied yarn where at least one strand appears loose and bulky compared to the other strands. Flashes may show up as a streak in a carpet.
There is a need for a system for making alternate ply twist yarns that will produce a package of yarn having a bond in the ply twisted yarn and a ply twist reversal at one end of the bond and having a uniform ply twist between bonds where the expected number of defects per 100 inch length of yarn is much lower than known alternate twist plied yarns, that have been found to have defect levels exceeding 5 defects/100 inch length. There is also a need for a high speed process for making alternate twist plied yarn where the defect level remains low.